The present invention relates to antenna tracking systems, and more particularly to a modified step track antenna system.
Highly directional antennas are used in the reception of the weak radio signals originated by satellites or other small or distant signal sources. To insure maximum signal integrity, the antenna must be oriented with the boresight axis (i.e. direction of maximum sensitivity) very precisely aimed at the signal source. Signal source motion complicates the problem and necessitates the provision of means for tracking the source so as to prevent loss of signal. Tracking systems employing error-sensing feeds, e.g. pseudomonopulse or conical-scan systems, can secure the desired result but are generally both complex and costly.
Step track antenna systems provide automatic tracking of satellites and other moving signal sources without the cost and complexity associated with these other antenna tracking systems. In step track systems, the antenna is aimed at the signal source through rotation in discrete angular steps about two orthogonal axes. The direction in which these steps are taken is determined during the interval between steps by comparing the then existing signal strength with the signal strength prior to the last step. If it is decided that the signal strength has increased, a further step is taken in the same direction as the previous step. If the signal strength has diminished, a step in the opposite direction is taken. A system of this general type is described in U.S. Pat. No. 3,842,420.
Rudimentary step track systems of this nature perform adequately in the tracking of slowly moving signal sources, but are not easily adapted for use in applications requiring tracking of rapidly moving signal sources. Some additional measure can be added to the maximum tracking rate by increasing the size or frequency of the positioning steps. Increases in the size of the steps, however, are necessarily accompanied by corresponding decreases in pointing accuracy. The maximum rate at which the steps can be taken, on the other hand, is fixed both by mechanical limitations and by the amount of time required between the steps in order to generate an accurate representation of the average signal strength. For these reasons, neither of the stated alternatives is entirely satisfactory.
This difficulty can, in large part, be circumvented by impressing a constant motion, hopefully approximating that of the signal source, onto the antenna boresight. The effect is to reduce the amount of boresight motion which must be accounted for by the step tracking process. This approach, however, only serves to increase the average rate of antenna movement and does not increase the dynamic range thereof. The antenna may still be incapable of automatically tracking signal sources displaying high angular dynamics as, for example, a satellite traveling in a highly elliptical orbit. Signal source acceleration may cause the boresight velocity to lead or lag the signal source velocity by more than can be compensated for by the step tracking process. When this occurs, the signal will be lost.